1. Field of the Invention
This invention relates generally to microphone assembly in a system which needs to convert sound waves to electrical signals.
2. Description of the Related Art
A microphone is a basic and essential element in any audio systems. There are many types of microphones in use currently. Generally, they are classified in four categories as listed in FIG. 1. The first one is an omni-directional microphone 102. It has a uniform polar response, i.e., the sound waves from any directions can be accepted and an electrical signal is generated with the same gain. A second type of microphone, a dipole microphone 104, can respond to sound waves mainly from two opposite directions. Sound waves coming from other direction have a much smaller gain. The sound wave coming from a direction that is 90° to the axis of the microphone element is not accepted, i.e. the gain is null. A third type of microphone is a cardioid microphone 106, which can accept sound waves from one primary direction. The response gain decreases as the incident angle of a sound wave deviates from the primary direction. The response gain drop can be substantial when the incident angle is greater than a microphone is a hyper-cardioid microphone 108. Hyper cardioid microphone 108 is like a hybrid of a dipole microphone and a cardioid microphone. It has a primary direction and a secondary direction, which is the opposite of the primary direction. It can respond to sound waves in both the primary and the secondary directions, but its gain for the secondary direction is less than the gain for the primary direction.
An array of microphones may also be assembled to emulate the properties of the above four types of microphones in some applications. For example, non-directional microphones may be grouped together. A controller may process the signals in such a way so as to generate a signal that is highly directional, so this array of microphones acts as if it is a directional microphone. Another example is discussed in U.S. Pat. No. 5,715,319, where several directional microphones are arranged in a circular array. The resulting microphone array acts similarly to a non-directional or omni-directional microphone. In this application, a microphone element can refer to a generic single element microphone, or a multiple-element-array, which behaves similar to a single element microphone. For example, a unidirectional microphone can be a single cardioid microphone, or a microphone array that accepts sound waves from a primary direction and rejects sound waves from most other directions. The microphone elements within the microphone array may be non-directional, bi-polar or hyper-cardioid or some combination.
Any one of the four types of microphones identified above has various disadvantages in audio systems, especially in audio conferencing and video conferencing applications. For example, an omni-direction microphone, which gathers sound from all directions equally, can be used in recording studios where the noise and reverberation level can be made to low, but gives poor quality in audio or video conferencing applications, because of its inability to reject reverberation and noise in a typical untreated room environment. A cardioid microphone only accepts sound waves directed towards the microphone and rejects most sound waves coming from other directions. This type of microphone may provide a higher signal to noise ratio (SNR) and a better sound quality, but it can only cover a very small area in the conference room. Participants in an audio or video conference may have to take turns speaking to the microphone. In some conference room setups, several such microphones can be connected to the system simultaneously, so most participants of the conference have a microphone nearby available to speak into. But this type of arrangement complicates the conference room and makes the room cluttered.
Although it is generally accepted that one may have to hold a microphone while giving a lecture in a large auditorium, it is still unnatural and inconvenient. In a conference situation, it is even worse. In an actual meeting, meeting participants would like to watch people's expressions on their face and other body language as they speak.
There are prior art devices that avoid many of the limitations of the microphone elements. For example, a Polycom SoundStation VTX-1000 speakerphone from the assignee of the current invention uses three microphone elements to provide better room coverage, SNR and frequency response. This speakerphone fulfills many requirements in a conference setting such that it appears on most conference room tables.
It is more desirable to eliminate the inconvenient microphones, or at least to keep them out of sight during a conversation and minimize their interference. It is desirable to have a microphone system that can provide coverage of the entire conference room, while at the same time keeping the sound quality high and maximizing the signal to noise ratio. It is desirable to have a microphone system that can provide other high quality sound processing.